The invention relates to a device for positioning sheets in a station of a sheet-processing machine, comprising a movable bearing surface for holding at least one edge zone of a sheet, actuating means for moving the bearing surface in the plane thereof, and gripping means accompanying the movement of the bearing surface and adapted to grip or release the said edge zone of a sheet between a sheet-gripping means and the bearing surface.
In machines such as multiple-station shaping presses for processing sheets of paper or full board or corrugated cardboard, the sheets must be very accurately positioned in both the longitudinal and the transverse direction in each of the stations through which they pass. Usually when a sheet is inserted into the machine, the front edge of the sheet is gripped by a conveying means such as a gripper bar comprising a number of grippers mounted on a transverse bar integral with side chains and subsequently conveying the sheet to the downstream processing stations of the machine, such as the cutting station or the waste-ejection station. The terms xe2x80x9cfrontxe2x80x9d, xe2x80x9clongitudinalxe2x80x9d, xe2x80x9ctransversexe2x80x9d and xe2x80x9clateralxe2x80x9d here are used with reference to the direction in which the sheets travel through the machine. In these stations, the problem of positioning the sheet is solved by positioning the shaping tools relative to the positions at which the gripper bar stops. The accuracy and productivity of the downstream processing stations will therefore depend on the accuracy of gripping.
The stacks of sheets are brought on a pallet and inserted into a feeder. The feeder ensures a regular supply of sheets to the feed board, which drives the sheets in a layer so that they can be gripped and conveyed to the downstream stations.
In currently-used positioning devices, the sheet on the feed table is aligned against front and lateral stops by feed means such as rollers or belts or elastic material on top, which come down and press against the feed board, or a pair of rollers disposed above and below the board. In the case of rough sheets, the front and lateral stops are disposed accurately with respect to the reference positions of the downstream stations. The sheet is rapidly pushed against the stops by the feed means, then gripped by a gripper bar. The stops are then retracted and the gripper bar can pull the sheet into the next processing station and position it accurately with respect to the tools on the work surface of the said station.
The problem of positioning is particularly troublesome when the sheet has undergone one or more previous processing operations such as printing or scoring the folding lines, since the subsequent operations must be performed very accurately with reference to the preceding processing. It is therefore advantageous to equip the sheet-positioning devices with systems for adjusting the position.
Patent CH 676 695 describes a positioning device in which the stops are motor-driven so as to control and vary their position and wherein a printed or other mark resulting from a previous processing operation can be detected by opto-electronic scanning means in order to adjust the position of the stops and consequently of the sheet, directly in dependence on the measured position of a said mark.
However, positioning devices with stops require each sheet to be practically immobilised during the entire aligning and adjustment operation thereof, thus greatly limiting the speed at which the sheets pass through the machine.
Patent application CH 0671/99 by the applicants describes a device for positioning sheets in the feed station of a sheet-processing machine of the kind defined hereinbefore, for operating the machine at much higher speeds. The sheet-gripping means is in the form of a transverse bar bearing lugs, all of which form a comb disposed above the bearing surface, with lateral uprights secured to the bar and each articulated at a single pivot point situated at the level of the movable bearing surface. The sheet-gripping means is controlled by a spring under pressure disposed between the bar and the bearing surface and tending to press the lugs of the sheet-gripping means against the bearing surface, and is also controlled by one or two control levers on the feed station, connected by a telescopic swinging rod to the rear of the bar and pushing and opening the sheet-gripping means by pivoting it.
The positioning process according to CH 0671/99 uses a horizontal bearing surface driven by actuators such as linear electric motors in longitudinal reciprocation and also in complementary corrective motion in longitudinal and/or transverse translation and/or in rotation around a vertical axis. This method, starting from the bearing surface in the starting or rear position, consists in engaging the sheet-gripping means on the bearing surface and then in controlling the actuators in order to move the bearing surface forward and if necessary sideways or crossways in dependence on the position co-ordinates of the sheet scanned by opto-electronic means during the advance of the bearing surface, so as to bring the front edge of the sheet in the grippers of a gripper bar into a precise, predetermined position, release the gripping means of the positioning device, and then return the bearing surface to the rear or starting position.
This method can substantially increase the productivity of the machine to up to 12,000 sheets per hour, whereas stop devices can only reach outputs of the order of 8,000 sheets/hour. However, this positioning device is not free from faults:
When the control lever or levers for opening the sheet-gripping means are put in to operation, the result is an impact and a consequent quasi-instantaneous force. Since the centre of inertia of the said means is separate from its pivot axis, the resultant of the forces has a horizontal component which affects the actuators. In particular, when the lever controlling the bar of the sheet-gripping means is on one side only, it pushes the bar via a rod and produces a torque which interferes with adjustment by rotation of the position of the sheet.
Even in the case where one control lever at each side acts on the sheet-gripping means, the push may cause a deflection, i.e. an error of up to a few tenths of a millimetre in the position at which the bearing surface stops. Such a deflection is disturbing, inter alia in the case where the sheet for processing is printed, when some outlines of the printing must coincide very accurately with the shape of the cut sheets.
Also, at the very high production rates made possible by this device, a recurrent horizontal component of a force when the sheet-gripping means is opened may interfere with the adjustment of the lateral linear motors, putting the position-correcting system out of action. In extreme cases there may be a resonance between the interfering force and the motors, resulting in jamming of the machine.
The aim of the invention is to eliminate these defects.
To this end, in a device of the kind defined in the preamble, the sheet-gripping means is connected to the bearing surface by at least one joint comprising a number of levers forming an articulated parallelogram.
In the device according to the invention, starting-up of the controls for opening the sheet-gripping means does not result in any appreciable interfering horizontal force capable of affecting the actuators for moving the surface bearing the sheet-gripping system.
The joint can comprise a fixed upright secured to the bearing surface and receiving an upper fixed pivot and a lower fixed pivot, disposed substantially one above the other.
Preferably the sheet-gripping means is connected to the pivot by an actuating lever and a return lever forming an articulated parallelogram.
More particularly the joint may comprise an upper actuating lever connected at its central part for rotation around the upper fixed pivot, its first end being rotatably connected to a lever for controlling the gripping means and its second end being connected to a movable upright, and the joint also comprises a lower return lever connected at its first end for rotation around the lower fixed pivot and connected at its second end for rotation around the lower end of the said movable upright, the said movable upright being integral with the sheet-gripping means.
Preferably a piston and spring permanently act on the lower return lever so as to press the sheet-gripping means against the bearing surface.
Of its nature, this kind of joint has a very small radial clearance at the level of the pivots and having an interfering effect on the actuators for moving the bearing surface at the moment when the sheet-gripping means is opened. To counteract this clearance, the movable upright integral with the sheet-gripping means is connected to the fixed upright by a piston and spring, the bottom end of which is rotatably connected to the said fixed upright and the top end of which is connected to the said moving upright. The spring piston is preferably permanently compressed.
Preferably the device comprises a joint at each transverse end of the sheet-gripping means, the said sheet-gripping means comprising a transverse beam and a number of lugs extending towards the front of the device and co-operating with the beam to form a first comb disposed above the front edge of the bearing surface, the said front edge of the bearing surface being profiled in the form of a second comb complementary with the first comb in order to nip the said edge zone of a sheet between them.
The beam constituting the sheet-gripping means and mounted on lateral uprights via a lever system forming articulated parallelograms on each side will normally have a slight lateral clearance between the components in relative motion, which may result in interference with the lateral motion of the bearing-surface actuators. To eliminate this lateral clearance, the device according to the invention can comprise at least one spring strip, flexible in a vertical plane and rigid in the transverse direction and connecting the sheet-gripping means to a lateral upright integral with the bearing surface.
More particularly the device can comprise a pair of spring strips made of polyester reinforced with glass fibre or more simply of spring steel, each fixed to a substantially horizontal plate integral with a said upright and also fixed to a plate integral with the end of the beam of the sheet-gripping means.